In radio communication equipment such as mobile phones or note type personal computers with a built-in radio communication function, a packaging density of parts is increased as the radio communication equipment is reduced in size. To address this, an antenna apparatus as disclosed in, for example, Japanese Patent No. 3758495 is proposed. In this antenna apparatus, a spiral conductor layer is formed on the surface of a base material made of a dielectric material or a magnetic material. A so-called chip antenna is installed on a substrate, and the chip antenna is grounded to a ground plane formed on the substrate. In this case, as shown in for example FIG. 5, an RF ground conductor 3 to which a chip antenna 2 on a substrate 1 is grounded ideally requires the most area on the substrate 1 in order to gain sufficient antenna characteristics.
Furthermore, in recent years, with the digitalization of radio communication equipment, a circuit ground conductor 4 to be used as a digital circuit ground has been formed on a substrate 1 together with an RF ground conductor 3 to which a chip antenna 2 is grounded, to thereby allow the coexistence of the RF ground conductor 3 and the circuit ground conductor 4 on the same substrate 1, as shown in FIG. 6.
However, the prior art described above has problems as follows.
That is, in a quarter wave antenna in which an electrical length of the antenna element is ¼ of a wavelength (λ), the size (especially, the length) of the ground plane to which the antenna is grounded is important. However, with a higher packaging density of parts, it has become difficult to secure a sufficient ground area necessary to obtain antenna characteristics in an ideal condition as shown in FIG. 5.
Furthermore, in the case where the RF ground conductor 3 and the circuit ground conductor 4 coexist on the same substrate 1 as shown in FIG. 6, there is a disadvantage that the ground plane on the substrate 1 is divided, making it impossible to secure the necessary area for the RF ground conductor 3.